More specifically, the embodiments of the present invention relate to a writing instrument comprising:                a barrel extending along a longitudinal axis between a rear end and a front end provided with an opening, and having a window in the a side of the barrel;        a writing tip that can move between an exposed position in which the tip emerges through the opening and a retracted position in which the tip is housed within the barrel;        a moving member connected to the writing tip; and        a side button designed to collaborate with the moving members which can be moved in a radial direction through the window between a first position for which the writing tip is retracted and a second position for which the tip is kept exposed.        
This type or writing instrument usually, but not necessarily is comprised of a ball-point pen, the side button of which has a clip so that the pen can be secured to a pocket of a shirt, for example. When the side button is in the second position it is generally pressed into the barrel of the pen, such that the clip cannot be used, which means the pen cannot be secured to a garment when the ball point is still exposed.
The writing tip needs to be kept exposed reliably and firmly enough for it not to retract when this is not wanted, particularly if the user exerts significant pressure on the tip or if he or she drops the pen onto a table or onto the floor. However actuation of the side button needs to remain simple and intuitive enough for the user that the writing instrument is practical to use and does not require particular dexterity or explanations as to how to expose or retract the writing tip.
For example, document EP A 0337447 discloses a retractable ball point pen of the aforementioned type, in which the side button, which comprises a clip, acts on the moving member equipped with inclined ramps in order to expose the writing tip. In order to keep the tip exposed, the side button has, at its front end, a tab 10a which becomes housed under a retaining part 13 so as to keep the side button in the pushed-in position. However, the locking thus achieved can be exerted only by exerting pressure on the front part of the side button, whereas it is necessary to exert pressure on the rear part of the side button in order to unlock and retract the tip. These special conditions for actuating the side button are not known to the user, which means that the writing instrument comprising such a mechanism may prove somewhat impractical to use. In addition, incorrect actuation of the side button may lead to deformation or wear of the tab 10a and of the corresponding parts of the moving member, which means that the locking achieved with an element that does not protrude very far may become unreliable and lead to unwanted retraction of the writing tip.